1. Field of the Invention
The present invention relates to a magnetic carrier, a two-component developer, a development method, a development device, and an image forming apparatus of electrophotography.
2. Discussion of the Background
In image forming apparatuses using electrophotography, such as copiers, facsimile machines, or printers, it is known to use a two-component development device having a two-component developer including magnetic carriers and toner or a single-component development device using only toner for development. Generally, a two-component development device includes a development sleeve serving as a developer bearing member. The development sleeve is cylindrical and is rotatably supported, and includes a magnetic roller inside thereof, the magnetic roller having a plurality of magnetic members with magnetic poles. A two-component developer including magnetic carriers to which toner has adhered is borne on a surface of the development sleeve to be conveyed to a development area formed between the developer bearing member and an image bearing member, wherein an electrostatic latent image borne on the image bearing member is developed with a magnetic brush formed by the two-component developer. In a two-component development device, magnetic carriers and toner are stirred and mixed, so that the charge property of the toner is relatively stable, and thereby a relatively stable and satisfactory image is obtained.
However, toner density in a two-component developer changes due to deterioration of magnetic carriers and consumption of toner in the developer, and the mixture ratio of the toner and the magnetic carriers of the developer changes. Therefore, generally, for suppressing a change in the mixture ratio of toner and magnetic carriers in a two-component developer, a toner density control device is provided, and new toner is replenished as necessary to suppress the change in the mixture ratio of the toner and the magnetic carriers.
In a single-component development device, toner borne on a surface of a developer bearing member is conveyed to a development area to develop a latent image borne on an image bearing member. Although certain drawbacks of a two-component development device, such as deterioration of magnetic carriers and necessity of providing a toner density control device do not exist in the single-component development device, the charge property of the toner is relatively unstable.
With respect to magnetic carriers used in such a two-component development device, it is generally desired that surfaces thereof are uniformly formed, and filming of toner on surfaces thereof, oxidization of surfaces thereof, and deterioration of the humidity sensing property are prevented. Further, a photoconductor serving as an image bearing member is desired to be protected from being scratched or worn by the carriers. Also, it is necessary to lengthen the life of a developer including the carriers, and to control a charge polarity of the developer or to adjust a charge quantity of the developer.
For those purposes, generally, a relatively firm and strong coating layer is provided to the carriers by coating the carriers with an appropriate resin material. For example, Japanese Patent Laid-open Publication No. 58-108548 describes a magnetic carrier coated with a resin material. Also, magnetic carriers including coating layers in which various types of additives have been added are described in Japanese Patent Laid-open Publications No. 54-155048, No. 57-40267, No. 58-108549, No. 59-166968, and No. 6-202381, and Japanese Patent Publications No. 1-19584 and No. 3-628, respectively. Further, Japanese Patent Laid-open Publication No. 5-273789 describes a magnetic carrier in which an additive adheres on the surface of the carrier. Also, Japanese Patent Laid-open Publication No. 9-160304 describes a magnetic carrier having a coating film in which conductive particles larger than the thickness of the coating film are contained. Japanese Patent Laid-open Publication No. 8-6307 describes a magnetic carrier in which benzoguanamine-n-butylalcohol-formaldehyde copolymer is used in major proportions for a carrier coating material, and Japanese Patent Publication No. 2683624 describes a magnetic carrier in which a cross-linking material of melanin resin and acrylic resin is used for a carrier coating material.
Also, for further enhancing durability of magnetic carriers, the present applicant proposes in Japanese Patent Laid-open Publication No. 2001-188388 an electrophotographic carrier having a coating film including at least a bonding resin and particles, in which a diameter D of the particles and a thickness h of a film of the bonding resin satisfies the relation: (1<D/h<5). In the proposed carrier, the particles are relatively convex as compared with the coating film. Therefore, in stirring a developer including the carriers and toner so that the developer is charged by friction, contacting of the carriers with each other or with toner, a strong shock against the bonding resin due to friction between the carriers or with the toner is mitigated. Thereby, excessive adhesion of toner to the carriers can be prevented, and at the same time scraping of the coating film of the bonding resin, where charging occurs, can be prevented, so that a change in surface shapes of the carriers over time is relatively small and durability of the carriers is greatly enhanced.
In the above-described two-component development device, with a recent demand for enhancement of image quality, a size of toner particles tends to be decreased, and concurrently with this, magnetic carriers also tend to be made small in particle diameter. Particularly, by making magnetic carriers small in particle diameter, a magnetic brush formed on a developer bearing member at the position where the developer bearing member opposes a photoconductor can be made relatively fine, and thereby enhancement of gradation in a halftone image and uniformity in a solid image can be expected. Further, because the magnetic carriers are made relatively light at the same time, it is advantageous to prevent deterioration of a developer including the magnetic carriers.
However, as the particle diameter of a magnetic carrier is smaller, magnetization intensity of the magnetic carrier is smaller, so that adhesion of the carrier to a photoconductor easily occurs. Generally, a magnetic carrier is held on a developer bearing member by a magnetic force, and at the same time, an electric charge due to electrostatic induction or charge injection exists in the magnetic carrier, and an electrostatic force acts between an electric charge on the photoconductor and that of the magnetic carrier. The magnetic force acting on each particle of the magnetic carrier is smaller as the particle diameter of the magnetic carrier is smaller. Therefore, when a magnetic carrier is small in particle diameter such that an electrostatic force of a photoconductor is greater than a magnetic force of a developer bearing member holding the magnetic carrier, the magnetic carrier easily adheres onto the photoconductor. Further, with a recent demand for miniaturization of an apparatus, the diameter of a photoconductor drum serving as an image bearing member and the diameter of a development sleeve serving as a developer bearing member tend to be decreased. With such miniaturization of the diameters of the photoconductor drum and the development sleeve, the magnetic holding force of a magnetic brush relative to carriers borne on ears of the magnetic brush at a downstream region of a development area formed between the photoconductor drum and the development sleeve (at the exit side of the development area) is decreased, so that adhesion of the carriers to the photoconductor drum as the image bearing member more easily occurs. With such occurrence of adhesion of the carriers to the photoconductor drum, deterioration of the photoconductor drum as the image bearing member, a cleaning blade for the photoconductor drum, and an intermediary transfer member is accelerated, and white spots in an image area and/or background soiling due to adhesion of the carriers to the photoconductor drum are generated in an image at the same time.
For preventing such adhesion of a magnetic carrier to a photoconductor, it is conceivable to increase magnetization of the magnetic carrier to increase a magnetic force of the magnetic carrier. In a ferrite carrier, however, the ratio of an iron component must be increased to increase a magnetic force of the carrier, so that the electric resistance value of a developer including the carrier is decreased. With respect to electrical resistance of developers and magnetic carriers, various studies have been made in the past. Japanese Patent Publication No. 2746885 specifies a range of dynamic resistance values of magnetic carriers when the magnetic carriers are conveyed by a developer bearing member. Japanese Patent Publication No. 2995949 specifies a range of volume resistance values of a developer including toner and magnetic carriers in a magnetic brush form in an electric field of 1000V/cm. By specifying lower limits of dynamic electric resistance values of a magnetic carrier and volume resistance values of a developer, charge injection from a developer bearing member to the magnetic carrier or charge injection from the developer to a photoconductor is prevented, and thereby adhesion of the carrier to the photoconductor, fogging in a background of an image, etc. are prevented. However, the above-described JP publications do not touch on address electrical resistance of magnetic carriers small in particle diameter.
As a method of remedying adhesion of a magnetic carrier to a photoconductor, it is conceivable to increase saturation magnetization of the magnetic carrier to a certain extent. By increasing saturation magnetization of a magnetic carrier, even when the particle diameter of the carrier is relatively small, the magnetic holding force of a magnetic brush relative to the carrier borne on an ear of the magnetic brush can be maintained to a certain extent. Saturation magnetization of a carrier has a certain relation with resistance of the carrier. When saturation magnetization of a carrier is increased, resistance of the carrier decreases, and on the contrary when saturation magnetization of a carrier is decreased, resistance of the carrier increases. However, it does not mean that a strict relation exists between saturation magnetization of a carrier and resistance of the carrier. Here, resistance of a magnetic carrier is so-called static resistance, which is a resistance value of the magnetic carrier measured a certain fixed time after a predetermined bias has been applied after having been put into parallel electrodes for resistance measurement and converted to volume resistivity.
If resistance of carriers is decreased, counter-charge remaining in the carriers after developing a solid image area easily deteriorates, so that adhesion of the carriers to an edge part of the solid image area, which is caused by the counter-charge, decreases. FIG. 1 is a schematic diagram illustrating states of an electric field of an image area and that of a non-image area. In the image area, an electric field, in which toner moves from a development sleeve toward the photoconductor drum side, is formed. In the non-image area, the electric field, in which toner moves toward the photoconductor drum side, does not exist. In an edge area E, which is a boundary between the image area and the non-image area, an edge electric field in which carriers move toward the photoconductor drum to adhere to the photoconductor drum, is formed. Intensity of the edge electric field is stronger as resistance of the carriers is higher, and is weaker as the resistance of the carriers is lower.
When resistance of carriers is relatively low, the above-described adhesion of the carriers to a photoconductor drum is decreased, but on the other hand an electric charge of the carriers easily leaks. In addition, when a superimposed bias in which an AC bias has been superimposed on a DC bias is applied between the photoconductor drum and a development sleeve bearing a developer including the carriers, because a relatively high voltage is instantaneously applied by the AC bias, the electric charge of the carriers leaks more easily.
If such conditions are combined, a leak occurs between the photoconductor drum and the development sleeve via the carriers, and thereby a latent image on the photoconductor drum is disturbed. As a result, density unevenness of a spotted pattern sometimes occurs in a halftone part of an image. A halftone image with density unevenness of a spotted pattern is herein referred to as a “spotted halftone image.”
Generally, electric resistance of a magnetic carrier is adjusted with resistance of resin for coating ferrite as a core member of the magnetic carrier. Experiments have been performed by inventors of the present application using a two-component developer including a magnetic carrier while adjusting electrical resistance of the magnetic carrier such that the dynamic electrical resistance value of the carrier and the volume resistance value of the developer are within the ranges specified in the above-described JP publications, respectively. However, a satisfactory result has not been obtained with respect to occurrence of the above-described spotted halftone image, and it has been found that a more detailed study on development characteristics of the developer in a development process is necessary.
Japanese Patent Laid-open Publication No. 10-55113 specifies a range of dynamic resistance values of a magnetic carrier in a magnetic brush form in an electric field of 104V/cm, which is close to a development electric field of an actual production apparatus. The JP publication describes that by setting the dynamic resistance value of a magnetic carrier within the specified range, adhesion of the carrier to a photoconductor, and an inferior image, such as the one an image having a brush mark resulting from breakdown of a latent image on the photoconductor due to bias leaking, can be suppressed, so that a halftone part of an image can be reproduced in high quality. However, the JP publication does not give any hint as to eliminating occurrence of a spotted halftone image.
Such a spotted halftone image may be avoided by setting resistance of magnetic carriers high to a certain extent. However, it has been found that sometimes an adverse effect occurs if resistance of magnetic carriers is increased such that generation of a spotted halftone image and adhesion of the carriers to a photoconductor drum can both be avoided. Specifically, an inferior image called a hollow image occurs, in which the periphery of a solid part or a character written in a halftone part thereof is dropped in white due to increase of the edge effect.
In a two-component development device, by using a magnetic brush to resemble an adjacent opposing electrode, a so-called returning electric field can be suppressed, so that it is possible to decrease the edge effect. Further, as a method of generating a state of an electric field similar to the one generated by bringing an opposing electrode closer, such methods are available as decreasing resistance of a magnetic carrier and decreasing a development gap. Accordingly, increasing resistance of a magnetic carrier as described above brings a state of an electric field similar to the one generated when an opposing electrode is separated in the distance, so that the edge effect is increased, and thereby a hollow image easily occurs.
As described above, it has been found that when taking measures to avoid adhesion of a magnetic carrier to a photoconductor that is caused by decreasing a particle diameter of the magnetic carrier, adverse effects are caused, such as occurrence of a spotted halftone image (a halftone image with density unevenness of a spotted pattern) and occurrence of a hollow image (an image in which the periphery of a solid part or a character written in a halftone part thereof is dropped in white). Thus, it is desired that adhesion of magnetic carriers to a photoconductor is suppressed and at the same time the above-described adverse effects are suppressed to a certain extent.